The present invention relates to a method for balancing a propeller shaft of the type which includes a slide type constant velocity joint, and more particularly relates to such a method for balancing such a propeller shaft including such a slide type constant velocity joint which is particularly applicable in the case that the slide type constant velocity joint has an internal cavity the volume of which is expanded and contracted as said slide type constant velocity joint slides in and out, said internal cavity being packed with grease.
A three part propeller shaft has been disclosed which has affinities with the propeller shaft disclosed in the present patent application as suitable for the application of the method of the present invention. Also in Japanese Laid-Open Publication Serial No. 59-38133 (1984), which it is not intended to admit as prior art to the present patent application except to the extent otherwise required by applicable law, three joint type propeller shafts are proposed of which the second and the third joints are fixed type constant velocity joints.
In order to reduce noise due to propeller shaft vibration, some passenger cars at the present time have come to be fitted with propeller shaft assemblies of a type incorporating at least one constant velocity joint of a slidable type instead of a Hooke joint, said slide type constant velocity joint acting as a rotational force transmitting means for coupling together two contiguous segments of the propeller shaft assembly. Such a slide type constant velocity joint usually has inside it a cavity the volume of which is expanded and contracted as said slide type constant velocity joint slides in and out, said internal cavity being packed with grease so as to provide good lubrication for the joint. The amount of grease typically packed into such an internal cavity is such that, when the slide type constant velocity joint is positioned to its so called neutral position in which it is pulled out to halfway between its fully slid in state and its fully slid out state, the cavity is filled to a prescribed packing ratio with grease, said packing ratio being substantially less than 100%.
The balancing of such a propeller shaft incorporating such a slide type constant velocity joint, however, has up till now presented certain problems. Typically, such a propeller shaft has been balanced by first positioning the constant velocity joint, as described above, to its so called neutral position in which it is pulled out to halfway between its fully slid in state and its fully slid out state, by then filling said cavity to the aforesaid prescribed packing ratio less than 100% with grease, by then mounting the propeller shaft either to a balancing machine or the actual vehicle on which it is to be used, with said constant velocity joint still in its so called neutral position, and by finally spinning said propeller shaft at high speed to determine its unbalance parameters so as to appropriately determine balance weights to be fitted thereto. Thus, in the prior art, such a propeller shaft incorporating such a slide type constant velocity joint has been balanced in a similar way to the way in which a conventional type of propeller shaft incorporating only Hooke joints has been balanced for a long time into the past.
A problem however with such a method for balancing such a propeller shaft incorporating such a slide type constant velocity joint is that during the process of spinning the shaft to balance it the grease received in the cavity of the constant velocity joint tends to become radially polarized, i.e. tends preferentially to accumulate at one position on the internal circumference of the cavity of the constant velocity joint. This is due to the fact that a considerable amount of air is present in said cavity as well as the grease, due to the fact that the aforementioned fill ratio is substantially less than 100%, and accordingly the grease is able to shift about towards one side of the cavity. This radial polarization of the grease means that an unpredictable unbalance effect due to the grease itself is engendered, and this in practice makes it impossible to balance the propeller shaft properly due to the unpredictable values of imbalance amount and imbalance phase produced by the off center grease accumulation or accumulations. This can cause undue propeller shaft vibration and noise. The problem of non uniform distribution of the grease is even further accentuated due to the fact that it is difficult for the grease to flow uniformly around in the cavity of the constant velocity joint, due to the high viscosity typical of such grease and due to the typically complex configuration of the interior parts of such constant velocity joints.